One type of head up display system for aircraft includes a cathode-ray tube (CRT) image source and a projection lens system that are mounted over and behind the pilot's head, and a stowable combiner that is positioned in a pilot's direct line-of-sight when the combiner is deployed for in-flight use. Visual flight information in the form of display symbology emanates from the CRT. Such information passes through the lens system and reflects off the combiner toward the pilot's eyes so that the display symbology appears to be coming from optical infinity. The display symbology is superimposed over the real world scene viewed by the pilot. The display symbology indicates, for example, the projected flight path of the aircraft ("aircraft symbology") relative to a fixed real world reference location, such as a runway. The aircraft symbology is projected toward a fixed location on the combiner in response to control signal information provided by a flight control system. The location of the aircraft symbology relative to the view of the runway observed through the combiner provides information which the pilot uses to control the flight path of the aircraft.
The combiner and the CRT must be spatially aligned within a preassigned operational tolerance to provide information to the pilot that accurately overlays the real world scene. Whenever the combiner and the CRT are misaligned by an amount exceeding the operational tolerance, display symbology will not conform to the real world, i.e., the locations of the aircraft symbology on the combiner will not correspond to the actual position of the aircraft relative to the runway. The resulting presentation of erroneous information without warning during, for example, aircraft landing manuevers may cause the pilot to direct the flight of the aircraft along an unsafe approach path to the runway.